Several changes:
* Allow bsd.man.mk to be included separately.
* Always include bsd.own.mk and bsd.obj.mk.
* Include bsd.man.mk and bsd.nls.mk even if NOMAN or NONLS; just turn off
building of the affected files instead.
* Require bsd.subdir.mk to be included explicitly.
(Will make appropriate changes to Makefiles shortly.)